The present invention relates to a method for the preparation of a silicone-based pressure-sensitive adhesive. More particularly, the invention relates to a method for the preparation of a silicone-based pressure sensitive adhesive having a high purity and stability of quality relative to changes in the viscosity and suitable for forming an adhesive layer on pressure-sensitive adhesive tapes, electrical insulation tapes, masking tapes for painting works, heat-insulation tapes and sheets and the like in an easily controllable way and with outstandingly low costs.
It is known that silicone-based pressure-sensitive adhesives of a typical class can be prepared by undertaking a siloxane rearrangement reaction between a first organopolysiloxane having a substantially linear molecular structure or a diorganopolysiloxane, e.g., dimethyl polysiloxane, represented by the average unit formula R.sup.1.sub.m SiO(.sub.4-m)/.sub.2, in which R.sup.1 is a substituted or unsubstituted monovalent hydrocarbon group and m is a positive number in the range from 1.8 to 2.1, and a second organopolysiloxane composed mainly of monofunctional siloxane units of the formula R.sup.2.sub.3 SiO.sub.1.5, in which R.sup.2 is a substituted or unsubstituted monovalent hydrocarbon group, and tetrafunctional siloxane units of the formula SiO.sub.2. Although the reaction can proceed even in the absence of any catalyst as is taught in Japanese Patent Publications 32-5099 and 33-2881, it is usual that a strongly basic compound such as a caustic alkali, e.g., sodium and potassium hydroxides, or an amine (see, for example, Japanese Patent Kokai 61-108682) is added as a catalyst to the mixture of the organopolysiloxanes because the process without using a catalyst must be performed at a high reaction temperature taking a long time with a consequently low productivity. The method using a caustic alkali as a catalyst has a problem because the alkaline catalyst after completion of the reaction can be removed completely only with extreme difficulties and any trace amount of the caustic alkali remaining in the product is detrimmental against the stability of the viscosity of the product to cause a decrease in the viscosity due to the alkali-catalyzed scission of the siloxane linkages. The remaining amount of the alkaline catalyst can of course be neutralized to avoid the detrimental effect of the alkali by adding an acid only to cause another problem, namely, that the precipitates of the salt formed by the neutralization can hardly be removed by filtration due to the very high viscosity of the adhesive. The problems are about the same as above in the use of an amine as the catalyst with the added problem that an adhesive product containing a trace amount of an amine compound is liable to cause discoloration. In short, the method of using a caustic alkali or an amine as the catalyst is disadvantageous in respect to the relatively low heat resistance as well as poor electrical properties of the product.
In this regard, an alternative method is proposed in Japanese Patent Kokai 61-108682 according to which a mixture of the two types of organopolysiloxanes is admixed with an organosilazane compound, e.g., hexamethyl disilazane, as a catalyst. This method is advantageous in respect to the stability of the viscosity of the product adhesive because the decomposition residue of the catalyst are, taking hexamethyl disilazane as an example of the organosilazane compound, trimethyl silanol and hexamethyl disiloxane having volatilizability with little possibility of remaining in the product after complete stripping. A problem in the use of a disilazane compound is that these decomposition residue are reactive with the organopolysiloxanes to cause a viscosity decrease so that it is sometimes difficult to obtain full reproducibility of the product quality depending on the balance between the dissipated and reacted amounts of the residue.